Black has entered an inferior variation of Petrov's Defence. This position is usually only seen among beginners. White's response is 4. Qe2, which threatens the black knight on e4.

If black decides to move the knight away, perhaps with Nf6, white will respond with Nc6+! which threatens black's queen and gives discovered check from the white queen down the e file. As black must move his king or block the check, the queen will be lost on the next turn with Nxd8 or Nxe7 if black moved Qe7 to block the check.

Thus, black's best move is not to move the knight. Attempting to protect it with d5 or f5 usually ends poorly, as the knight is pinned and white can pile on attackers with f3 and d3. In practice, if black finds himself in this situation, he will most likely lose the knight.